1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and a semiconductor device manufactured by the manufacturing method. The semiconductor device described here includes an electro-optical device such as a liquid crystal display device and a light emitting device, and an electronic device using the device as a display portion.
2. Description of the Related Art
In recent years, there has been widely used a technique of crystallizing an amorphous semiconductor layer formed on an insulator, more specifically, a glass substrate to obtain a crystalline semiconductor layer and producing thin film transistors (hereinafter referred to as TFTs) using the crystalline semiconductor layer as an active layer, and an electrical characteristic of the TFT has been markedly improved.
Therefore, various signal processing circuits conventionally externally mounted using an IC and the like can be produced using the TFTs so that a display device in which a pixel portion and driver circuits are integrally formed on a substrate is realized. By a reduction in the number of parts, display devices are compact and lightweight, and further a manufacturing cost can be greatly reduced. Recently, research and development have been widely advanced.
As one of methods of preferably crystallizing an amorphous semiconductor layer, there is a technique of irradiating CW (continuous wave: continuous oscillation) laser light to the semiconductor layer while being scanned in a single direction so that crystals are grown in a scanning direction to form single crystals lengthened in that direction. When this method is employed, it is considered that a layer is obtained which includes few grain boundaries at least in a channel direction of the TFT. Further, the respective grain boundaries have a composition close to single crystal so that an electrical characteristic and uniformity thereof are superior.
According to this method, in order to irradiate laser light to the semiconductor layer while its energy density is continuously kept to a sufficient magnitude, laser light is linearly condensed and irradiated. Therefore, when the entire semiconductor layer on the substrate is crystallized, a laser light spot is shifted relative to the substrate to scan it on the substrate, thereby conducting laser irradiation. Thus, a point in which it takes a long time for processing becomes a problem. Note that a spot of laser light is accurately made to an elliptical shape or a rectangular shape. However, its aspect ratio is large. Thus, it is defined to be a line shape.
Also, as means for improving the crystallinity of the semiconductor layer, there is means to which the fundamental principle of graphoepitaxy is applied. This is to utilize the anisotropy of surface energy which growing crystal has, that is, a property in which all orientation surfaces are not randomly produced in crystal growth but bias in orientation is caused in a specific condition.
As shown in FIG. 1A, when Si is grown on a smooth amorphous quartz substrate or an SiO2 film 201, a (100) orientation surface is easy to produce on a surface which is in contact with the substrate. However, there is no regulating element with respect to an in-plane orientation so that the orientation becomes arbitrary. Therefore, as shown in FIG. 1B, a slit-shaped unevenness portion made from a base insulating film 202 is provided on a smooth surface and crystal growth is made thereon. At this time, because Si is in contact with the bottom surface, the top surface, and the side surface of the slit, it is grown such that a (100) orientation surface and a (010) orientation surface and a (001) orientation surface which are equivalent thereto are in contact with the respective surfaces thereof. Accordingly, single crystals in which in-plane orientations vertical to and parallel to the substrate are aligned are grown. Similarly, as shown in FIG. 1C, when a slant slit-shaped unevenness portion is formed and crystal growth is made thereon, single crystals having a (110) orientation surface can be also obtained. Details on crystal growth based on this principle are described in, for example, “Crystalline Silicon on Insulators by Graphoepitaxy (1979, IEEE page 210–213)”.
In FIG. 2A, a process of forming a base layer 1102 on a substrate 1101, subsequently forming a semiconductor layer 1103 thereon, and crystallizing it to obtain a crystalline semiconductor layer is considered. When the semiconductor layer formed over the substrate is crystallized, stress resulting from crystallization is produced within the layer. There is a time difference in in-plane crystallization. Thus, a stress distribution does not become uniform so that a point on which the stress concentrates appears. For example, a stress distribution as shown in FIG. 2A is caused in a grain boundary between crystal grains 1105 and in the vicinity thereof so that a surface which would originally become a smooth surface indicated by reference numeral 1104 has a shape in which the center and its vicinities of crystal grain rises. In addition, there is the case where cracks are caused as indicated by reference numeral 1106.
By the way, a driver circuit of a display device will be described as an example of a semiconductor device. FIG. 3A shows an example of a general display device. It has a pixel portion 503, a source signal line driver circuit 504, a gate signal line driver circuit 505, and the like on a substrate 501, and the resultant substrate 501 is bonded to a counter substrate 502. A signal from the outside is inputted through a flexible print circuit (FPC) 506.
FIG. 3B simply shows a configuration of the source signal line driver circuit of the display device. It has a shift register 511 composed of a plural stages of D-flip-flops 512, NANDs 513, buffers 514, sampling switches (analog switches) 515, and the like.
In accordance with clock signals (CLK and CLKb) and a start pulse (SP), sampling pulses are outputted from the shift register 511 in succession. Subsequently, they are shaped to pulses which are not duplicated between adjacent lines by the NANDs 513, passed through the buffers 514, and then inputted to the sampling switches (analog switches) 515. The sampling switches (analog switches) 515 are turned ON at timings when the sampling pulses are inputted thereto and video signals at respective times are captured to the source signal lines (S1, S2, . . . , Sn).
Here, the sampling switches (analog switches) 515 each have a large load (here, a wiring resistor, a capacitor, or the like of the source signal line). Thus, in order to provide sufficient drive capacity, a channel width (W) is increased. Further, even in the buffers 514, in order to drive the sampling switch having a large W, W in a post-stage is increased. Its size is changed according to the amount of load but is usually several hundreds of μm.
With respect to a TFT using a semiconductor layer formed on an insulating substrate as an active layer, variations in electrical characteristics resulting from in-plane nonuniformity of semiconductor crystals becomes a problem. In particular, in the case of a TFT having a large size, a degree of variations thereof cannot be neglected. Further, when the characteristics of the sampling switches and the like are varied, video signal capture operation is influenced thereby. Thus, a display quality is reduced so that it becomes serious.